Ionization gauge circuits



G. R. NAGEL ETAL 2,455,437

4IONIZA'l'I-ON GAUGE CIRCUITS Filed Nov. 13, 1945 Patented Dec. 7, 1948 uNirD STATES PATENT 'ortica roNIzATIoN GAUGE cmcm'rs Gerhard R. Nagel and Merriam E. Johnson,

Rochester, N. Y., assignors to Distillatlon Products, Inc., Rocheste Delaware r, N. Y., a corporation o! Application November 13, 1943, Serial/No. 510,186

1 claim. (ci. ris-183) 1 This invention relates to ionization gauges, and more particularly to an improved system of electrical circuits and devices for quickly and accu` rately determining the absolute pressure measured by such gauges.

An ionization gauge is an instrument formeasn uring low subatmospheric gas pressures, of the order of 10-4 millimeters of mercury or less, by

measurement of the positive ion current flow in.

tween cathode and anode. This electron current flow ionizes the atoms or molecules of gas in the region its transverses, and most of the positive ions so formed ilow to the collector plate. -For gas-pressure below about 10-4 millimeters of mercury, this positive ion current is directly proportional to the gas pressure in the tube and to the magnitude of the electron current flowing from cathode to anode at any given value of anode potential. With a fixed and predetermined electron current flow, the flow of ion current between the collector plate and the cathode may thus be used as a direct index of the gas pressure within the tube.

'Ihe positive ion current ordinarily ranges in value from a fraction of a microampere to 100 microamperes or more. Accurate measurement of this current without disrupting the factors that produce it and without the use of expensive indieating instruments and other adjuncts is a dil cult problem. It is the object of the present invention to provide a system comprising a combination of electrical circuits and devices for rapidly and accurately measuring the positive ion current of an ionization tube and so directly determining the absolute pressure of the gastherein. A further object is to provide a system of this nature in which variations in voltage employed to operate the circuits are cancelled out rather than magnified whereby the system may be operated without the use of standard cells, voltmeters or like expensive adjuncts. Another object is to provide a circuit in which the various voltage adjustments incident to operation and calibration can be made independently and without troublesome changes in voltage in other parts 2 of the system. The invention further provides a system of the type described in which ordinary commercial vacuum tubes may be -used without pre-selection and without impairment of the accuracy of operation. Various otherv specic objects'and advantages of the invention will be apparent from the following description of a typical embodiment thereof.

In describing the invention in detail. reference will be made to the accompanying drawing in which the single gure is adiagrammatic and simplified representation of the circuits and devices making up a systenpembodying the'present invention.

In the drawing, IT represents an ionization tube comprising an envelope I enclosing a laof conventional design.

ment or cathode 2, an anode 3 and a positive ion collector plate 4. The envelope I is provided with a connecting duct 5' by means of which the gas pressure to be measured may be imposed on the tube. The cathode 2 is connected through a center tapped resistor 6 and wires 1 and 8 with a tap on the voltage dividing resistor 9 of a source of unidirectional potential.

As shown, unidirectional potential is applied across the resistor 9 by 'a vacuum tube rectier This includes a power transformer TI energizing the plates and lament circuits of a full wave rectifier tube I0, the output circuit of which includes a choke coil II and bypassv condensers I2. A switch I3 contr-ois the application of alternating current to the primary of the transformer TI from a suitable source which may be a volt 60 cycle power line.

The anode 3 of the ionization tubeIT is arranged to be heated for degassing, and is accordingly provided with leads I4 and I5 connected to its opposite ends. These leads run to a double pole double throwswitch I6 which in its upper position connects the anode 3 through lead I5 to the wire I'I, and in its lower position connects the leads I4 and I5 across a degassing current circuit ZZ to be described. The anode 3 is maintained at a positive potential with respect to the cathode 2. To this end, the anode 3 is connected through the lead I 5, the switch I6, the wire Il, the switch I8, the resistor I9 and the wire 2U to a point on the potential source resistor 9 which is positive with respect to the point at which the cathode 2 is connected thereto.

The positive ion collector plate 4 of the tube IT is provided with an ion current circuit including a source of potential for maintaining the plate 4 slightly negative with respect to the cath- Aode 2. As shown. the plate 4 is connected through the wires 2I and 22. one ot the resistors RI, R2 or R3 of an adjustable resistance unit R, the switch arm 23, the wire 24, movable contact 29,

a portion of the variable resistor 25 and the wire 28 to a point on the resistor 9 which is negative 5 with respect to that vat which the-cathode 2 is connected to this resistor.

In accordance. withy the invention, the voltage set up by theow of positive ion current through the resistance unit R is opposed by a voltage of opposite polarity set up across the resistor 25, and

the portion of the resistor included in the circuit is varied until these voltages are equal, whereupon the magnitude of the positive ion current and thus the gas pressure within the ionization tube can be determined from the settings of the adjustable resistance unit R and the movable contact 29 of the variable resistor 25. One end of the variable resistor 251s connected by the wire 26 to a point on the potential source resistor 9 '2o near its negative end, and the other end of the resistor 25 is connected through a variable caliy brating resistor 21 and a wire 28 to the negative end of the resistor 9. The movable contact 29 of the resistor 25 is provided with a suitable scale 25 30 for indicating the portion of this resistor included in the positive ion current circuit. By suitable adjustment of the variable resistor 21 and choice of a suitable scale 30, movement of the pointer 29 may be arranged to directly indicate 30 the gas pressure'in the ionization tube under standard conditions' of electron current flow therein.v A suitable visual indicator, preferably. a cathode ray tube CRI, is employed to indicate adjustment of the variable resistor 25 at the point 35' where the opposite voltages in the io'n current circuit are equal. A bypass condenser 3l is con'- nected across the resistors R, 25 and 21. The cathode ray tube CRI is of a known type in which the effects of a change in controlling voltage applied to the gridfcathode circuit is visually indicated on a uorescent target. The construction and operation of such tubes is well known in the art and will not be described in detail. It is suilicient to explain that the luminous target is substantially circular and that a control electrode produces a shadow angle thereon which decreases as the grid potential changes in the negative direction and widens as the grid potential changes in the positive direction.

The grid-cathode circuit of the'tube CRI is connected across that portion of the positive ion circuit which contains the series connected resistances R and 25. Thus the grid 32 of the cathode ray` tube is connected by a wire 33 to the wire 22 55 'and the cathode 34 thereof is connected through a variable biasing resistor 35 and a wire 35 to the wire 29. Positive potential is applied to the plate 31 of the cathode ray tube by a wire 33 connected to the potential source resistor 9.

In order that the positive ion current of the tube IT may accurately indicate the gas pressure therein, the electron current from cathode 2 to anode 3 must be maintained at a predetermined value. This electron current is adjusted by vary- 05 ing the cathode heating current, and adjustment to the desired value is indicated by a. second cathode ray tube CR2 of the same type as that described above. A transformer T2 has a primary winding 39 connected in parallel with the pri- 'i0 mary of the transformer TI. The secondary winding 40 of the transformer T2 is provided with a plurality of taps 4I, selectively engageable by a switch arm 42 connected to one terminal of a cathode current switch 43. The other terminal 'I5 of the switch 43 is connected to one end of the transformer secondary 40 by a wire 44. Degassing current forfthe ionization tube anode 3 is supplied from the 4terminals ZZ connected respectively to the wire 44 and one of the secondary taps 4I. The cathode current switch 43 is connected to the cathode terminals YY through a rheostat 45. A pilot light 45 may be connected across the cathode current circuit as shown.

Heater current is supplied to the cathode heater terminals XX of the cathode ray tubes CRI and CR2 from a secondary 41 on the transformer TI, and a pilot light 48 may be connected in parallel with this supply as shown.

The grid-cathode circuit of the tube CR2 is connected across the resistor I9 in the ionization tube anode circuit, and a variable grid biasing resistor 49 is included in the cathode lead 50 as shown. Positive plate potential for the tube CR2 is sup plied by the wire 5I connected to the positive end ofthe potential supply resistor 9. By throwing the switch I5 to its lower position, the grid-cathode circuit of the tube CR2 is connected across a portion of the potential supply resistor 9 through a fixed resistor 52 and the wires 53, 9 and 20.

-In employing the describedA embodiment of our invention to measurement oi gas pressures, the

ionization tube IT is connected through the duct 5 to a vessel containing the gas under pressure to be measured. 'Then with the cathode current switch 43 open, the rheostat 45 moved to its maximum resistance position and the arm 42 moved to the lowest'of the taps 4I, the power supply switch n 1s dosed. After thennments ofthe tubes lo, CRI and CR2 have' warmed up, the cathode current switch 43 is closed and the switch arm 42 is moved to successively/.higher taps 4I until the ionization tube cathode 2 lights. The cathode current is gradually increased by the rheostat 45 until the shadow angle of the tube CR2 decreases to the minimum, which indicates that the electron current in the anode circuit of the ionization tube is at the desired predetermined value. This indication results from the connection of the grid circuit of the tube CR2 across the resistor I9 in the anode circuit of the ionization tube. The grid biasing resistor 49 of the tube CR2 is adjusted as hereinafterA described so that the minimum shadow angle indication of this tube corresponds to the desired anode circuit current.

The movable contact 29 of the variable resistor 25 is moved to the left to its zero position at which all of the resistor 25 is'cut out of the grid circuit of the tube CRI. The resistance unit R is set to include the resistor Ri of highest resistance in the positive ion current circuit. Positive ion current ilows from the collector plate 4 to the cathode 2 of the ionization tube IT through the resistor RI. This current ilow produces a voltage across the resistor RI which opposes the voltage across the grid bias resistor 35, thus reducing the negative grid bias on the tube CRI and causing its shadow angle to increase. The contact 29 of the resistor 25 is now moved to the right, introducing into the cathode ray tube grid circuit a voltage of opposite polarity to that developed across the resistor RI by positive ion current iiow. This opposing voltage is adjusted by movement of the contact 29 until the shadow angle of the tube CRI decreases to the minimum, which indicates that the net voltage imposed 4on the grid circuit of this tube by the flow of positive lon current is zero. The settings ot the resistance unit R and the variable resistance contact 29 now indicate the magnitude of the positive ion current, and by proper calibration may be made to directly indicate the pressure of the gas in the ionization tube IT.' Thus for example with the circuits properly adjusted, the resistor Ri may be designated l and the position of the contact v29 on the scale 32 may be for a minimum shadow angle. on CRi, and in this case the pressure of the gas in the ionization tube would be 5X lil-6. If the minimum shadow angle cannot be obtained with full scale movement of the contact 29, the switch arm 23 of the unit R is moved to insert the resistor R2 of the next lowest resistance, and the movable contact 29 is adjusted for minimum shadow angle. The variable resistor 2l may be adjusted so that the position of the contact 29 on the scale 32 indicates gas pressures directly.

For proper operation, the grid bias of the cathode ray tube CB2 must be so adjusted that a minimum shadow angle thereon indicates the desired predetermined electron current inthe anode circuit of the ionization tube IT, and the grid bias of the tube CRI must be so adjusted that a minimum shadow angle on this tube indicates zero net voltage across the.. portion of the positive ion current circuit common to the grid circuit of CRI. To make theiirst of these adjustments, the cathode current switch 43 is opened, the switch i8 is moved to its lower position and a current limited by the resistor 52 to the magnitude desired in the anode circuit ows through the resistor i9 across which thegrid circuit of thetube CB2 is connected. The grid bias resistor i9 is then adjusted until the shadow angle of the tube CB2 is reduced to a minimum. Thereafter, when anode circuit currentflows through the resistor i9, a minimum shadow angle `on CB2 indicates that this current isv value. Y

To adiust the grid bias resistor of the tube CRI the switch 43 is opened so that no positive ion current ows, the movable contact 2B is moved to itsextreme `left hand position to cut all ofthe resistor 25 out of the grid circuit and the biasing resistor 35 is tiren adjusted until the shadow angl of the tube CRI is reduced to the minimum.

adjusted' to .the desired working From the foregoing description, it will be' apparent that by the use of our improved-system low subatmospheric gas pressures may bedirectly, accurately and quickly determined without the use of expensive meters orstandard voltage cells. Due to the arrangement of the circuits, min'or variations in operating voltages due to the usual fluctuations in line voltage tend to cancel out l Y in the positive ion circuit by the resistor 2i. Eachof the several voltage adjustments required for ,s A zero setting of the cathode ray tubes, adjustment of the anode current of the ionization tube and pressurel determination are made without disturbing the voltage distribution in other parts of the system, whereby the adjustment and calibration of the system is simplified. The cathode ray tubes may be operated at a grid bias corresponding to extreme low grid current and accordingly measurements of pressures as low as 10"" millimeters current in said anode circuit, a positive lon current circuit connecting said collector plate and said cathode and including in series a resistance unit, a variable source or unidirectional potential having a polarity opposite to that of the voltage generated across said resistance unit by the iiow of positive ion current therethrough, two cathode ray tubes each having a grid circuit, means for connecting the grid circuit of one oi said cathode ray tubes across the resistor in said anode circuit and means for connecting the grid circuit of the other of said cathode ray tubes across the portion of said pitive ion current circuit including said resistance unit and said variable source of unidirectional potential.

' GERHARD R. NAGEL.

MERRIAM E. JOHNSON.

nnrnnmscns crrnn UNITED STATES PATENTS Number Name Date Buckleyr i. Mar. 29, 1921 vBuckley Nov. 15, i927 Jackson Oct. 24, 1933 Rosebury Nov. 16, 1937 Wagner June 28, 1938 Jackson Aug. 1, 1939 Shepard Mar. 18, 1941 Instruments, 0ctober1934, pp. 205-208.

Electrical Times, September i7, 1942, pp. 40o-402. Wireless World. July` 1943, pp. mi2-204;

-.Dushman Mar. 16, 1920,

' King l Dec. 22, 1925 Moles Aug. 7, 1934 

